This invention relates to improved electrical methods and apparatus for heating the soil or subsurface formations to remove volatile or semi-volatile contaminants or petroleum substances. The electrical heating of sites containing such hydrocarbonaceous or thermally responsive materials generally requires the use of oppositely polarized, equally spaced electrodes of about the same length embedded vertically into the formation. Such electrodes are frequently powered by multi-phase power frequency sources comprising various combinations of Wye and Delta transformers. In this invention, arrangements and designs of electrodes are disclosed which suppress current pathways which would tend to bypass the formation to be heated. By such suppression, greater efficiency is achieved in heating, in contaminant or product recovery and in economic operation.
One application of this invention will be to more effectively and efficiently assist in the removal of hydrocarbon-like contaminants, particularly volatile contaminants such as gasoline and TCE. Another application of this invention would be to assist in the in situ extraction of various thermally responsive minerals such as heavy oil or sulfur.
One current method of decontaminating sites contaminated by hydrocarbons, especially for those containing semi-volatiles and high boilers, comprises excavating the entire site and burning all excavated site materials in a portable incinerator. Such a method becomes exorbitantly costly if the site is extensive and may be impracticable if the contaminated site is deep.
Another system has been developed which applies reduced pressure or a vacuum to the formation for in situ remediation of soils contaminated with volatile hydrocarbons such as gasoline or trichlorethylene. This is generally referred to as vacuum extraction technology, abbreviated as VET. This involves drilling a well into the vadose zone of the earth and applying a vacuum via the wells to volatilize and collect the contaminants. Multiple wells are used for large contaminated sites. Injection wells are sometimes used in combination with recovery wells. A common drawback is the inability to economically treat sites containing less volatile materials such as jet fuels. Another difficulty is the relatively long period of time required to extract the contaminants with a high degree of recovery. Yet another limiting factor is the high cost of treating the effluent materials which may contain hazardous components, because of the dilute nature of the contaminants in the effluent stream. Such systems are described in U.S. Pat. No. 4,183,472 to Knopik, U.S. Pat. Nos. 4,593,760 and 4,660,639 to Visser and Malot, U.S. Pat. No. 4,442,901 to Zison, and U.S. Pat. No. 4,730,672 to Payne, which all disclose variations on the vacuum extraction system. In all these cases, the formation is processed at ambient temperatures.
One way mitigate the difficulties with the VET method is to volumetrically heat the formation of interest. This can be done electromagnetically either with low frequency ohmic heating, or high frequency dielectric heating.
Electromagnetic or radio frequency (RF) heating of earth media or formations containing hydrocarbons or noxious volatile wastes has been the subject of investigation over the last 10 to 20 years. The objective has been to heat the formation to assist in the removal of valuable materials such as oil, or noxious materials such as solvents and liquid fuels. Current in situ electromagnetic heating technology falls into two major categories: A) bound-wave heating (either low or high frequency), and B) radiated wave heating (high frequency only).
Bound-wave heating structures are those in which the wave is largely contained within a specified volume and is not permitted to radiate significant amounts of energy. The original purpose of radiated wave structures (antenna), on the other hand, was to radiate waves into a lossless dielectric, such as air. Examples of the bound-wave approach appear in U.S. Reissue Pat. No. 30,738, and in U.S. Pat. Nos. 4,140,180, 4,144,935, 4,499,585, 4,498,535 and 4,670,634. The successful application of the bound-wave process using the high frequency version is discussed in "Development of the IIT Research Institute RF Heating Process for In Situ Shale/Tar Fuel Extraction-An Overview", presented at the Fourteenth Oil Shale Symposium, Colorado School of Mines, Golden, Colo., April 1981 by R. D. Carlson, et al. The successful use of a high frequency version of the bound-wave heating to decontaminate hazardous waste spills appears in "Radio Frequency Enhanced In Situ Decontamination of Soils Contaminated with Halogenated Hydrocarbons", presented in the proceedings of the Twelfth Annual Research Symposium, U.S. EPA, Apr. 21-22, 1986, U.S. EPA Publication No. EPA/600/9-86/022 by H. Dev.
Direct application of radiated wave technology to heating lossy media such as soil has not achieved the same degree of success as bound-wave methods. Examples of direct application of antenna technology, intended for radiation in lossless media such as air, to heating lossy media appear in U.S. Pat. Nos. 4,301,865, 4,140,179, 4,457,365, 4,135,579, 4,196,329, 4,487,257, 4,508,168, 4,513,815, 4,408,754, 4,638,863, 2,757,738, 4,228,851, 3,170,519, and 4,705,108.
The lack of reported success in using the radiated wave approach in highly conducting earth media (as opposed to air) may be attributed to several possibilities. One possibility is that far field radiated wave technology, which was originally developed for radiation into lossless media such as air, has been incorrectly adapted for media which are highly conducting. Another possibility originates in the misconception that hydrocarbon material can be selectively heated to high temperatures, regardless of the soil matrix, even though such material is both finely divided and widely dispersed in the matrix. Such a misconception may have led to impractical equipment and negative results. An example of a radiating antenna structure designed to recover hydrocarbons (either contaminants or fuels) embedded in lossy earth is described in U.S. Pat. No. 5,065,819.
In these high frequency methods, electrical energy is applied to the formation to heat the formation volumetrically, preferably as uniformly as possible. Such heating increases the vapor pressure of the contaminants. If enough heat is supplied, water will also be evaporated to create a steam sweep which further enhances contaminant removal. The volatilized contaminants are collected in a tent-like vapor barrier over the site, and then the contaminants are removed from the vapor stream. Such systems have been disclosed in U.S. Pat. No. 4,670,634 to Bridges, et al., and are further described in the publication by H. Dev entitled "In Situ Radio Frequency Heating Process for Decontamination of Soil" presented in Solving Hazardous Waste Problems presented at the 191st meeting of the American Chemical Society in April 1986.
The foregoing references describe in situ heating systems which are designed to vaporize the water in order to extend the range or reach of the heating pattern. Another objective of vaporizing the water is that it creates a steam sweep system wherein the presence of steam dries out hydrocarbon contaminants which have a boiling point well in excess of the temperature of the formation. For example, where approximately 25 tons of the formation was heated to a temperature of 150.degree. C., it was possible, by the use of a steam sweep, to remove nearly 80% of the hydrocarbon contaminants with boiling points near 300.degree. C. or above. Such systems can effectively remove semi-volatiles such as diesel and jet fuel and high boilers such as PCBs and PCPs.
There are presently also large volumes of earth that are contaminated with gasoline, TCE or carbon tetrachloride. These are considered volatiles, with boiling points close to that of water. Other methods may be used in these circumstances. One method to extract volatiles comprises forcing air through the formation to cause the volatiles to evaporate. The difficulty with this particular process is that it takes a great deal of time to extract the volatiles at normal earth temperatures. Also, the hydrocarbon concentration of the volatiles in the effluent stream is quite small, thereby complicating the effluent treatment process.
Heating the formation to temperatures somewhat below the vaporization point of water enhances the rate of hydrocarbon extraction. This will have at least two major advantages. First, the rate of evolution of hydrocarbons is increased, and second, the concentration of hydrocarbons in the effluent stream is not diluted by water vapor, thereby decreasing the cost of effluent treatment.
One method to heat the formation would be to use high frequency energy as previously discussed. However, while this may be cost effective when higher temperatures are required, it can be costly inasmuch as the conversion of AC energy to RF energy exhibits some loss of efficiency and the RF generator and related high frequency equipment represent high capital costs.
Alternatively, 60 Hz ohmic heating may be used to heat such a formation. In such circumstances, electrodes are placed in the formation and excited by 60 Hz voltages. These cause 60 Hz currents to flow through the formation, thereby heating the formation. Depending on the nature of the electrodes, row spacing and heating time, it may be possible to heat the formation sufficiently, if not uniformly, throughout so as to make the in situ venting or air stripping system far more effective.
One high temperature method is described by Brauns, et al. in U.S. Pat. No. 4,376,598, by Buelt, et al. in U.S. Pat. No. 4,957,393, by Timmerman in U.S. Pat. No. 5,024,556, and Carter in U.S. Pat. No. 5,004,373. These inventions are designed to heat the soil well above 100.degree. C., the vaporization point of water. Two or more electrodes are installed vertically around the contaminated soil to be treated. By means of non-aqueous but highly conductive material placed on the surface of the soil or within the soil itself, conduction currents between electrodes excited by 60 Hz potential are maintained until the soil itself becomes sufficiently conductive at temperatures well above 300.degree. C. to maintain significant current flows. No considerations are given to optimizing the systems to efficiently heat the soil at temperatures below 100.degree. C. or to remove organic-like volatile or semi-volatile contaminates at temperatures below 100.degree. C.
Other examples of the use of 60 Hz ohmic heating of earth formations are found in Perkins U.S. Pat. No. 3,958,636, Pritchett in U.S. Pat. No. 3,948,319, Todd in U.S. Pat. No. 4,084,637, and Kern in U.S. Pat. No. 4,010,799. All disclose methods to heat subterranean formations by means of emplaced electrodes excited by 60 Hz energy. All of the foregoing inventors describe the use of sparsely-spaced, vertically-emplaced electrodes, where the length of each electrode is considerably smaller than the spacing between the electrodes. However, many novel ways of exciting these electrodes are presented. For example, Pritchett describes a method of electrical multi-phase excitation of electrodes in triangular or hexagonal arrays.
Such sparsely-spaced electrodes result in overheating near the electrodes and underheating midway between the electrodes. Dense arrays of rows of vertical electrodes have been developed and tested which mitigate the excessive over-and-underheating effects associated with sparse arrays. A dense array is defined as an array in which the separation between rows is larger than the separation between electrodes within the rows. An additional feature of such dense arrays is that the extent of the rows of excited electrodes may be symmetrically less (both transversely and longitudinally) than the extent of the grounded rows of electrodes. Such a dense array excited by power frequency sources is described by Bridges, et al. in U.S. Pat. No. 4,545,435 and this approach is a special case of the nearly uniform heating approaches described by Bridges, et al. in U.S. Pat. R.E. 30,738. Limiting the extent of the exciter row dramatically reduces the flow of currents and related heating into formations of no interest.
However, in some situations, the use of oppositely polarized electrodes of about the same length, spaced equidistant from one another may be advantageous. Yet, methods and equipment to heat earth formations by such electrode arrangements without excessive temperature disparities are lacking. A major difficulty exists with sparsely-spaced electrode arrays where the spacing between the electrodes is substantially greater than the length of the electrode. Under these conditions, the mutual impedance between the electrodes is small or virtually non-existent and, as a result, each electrode acts as an independent source of heat which only heats the soil immediately surrounding the electrode. Under such conditions the charge distribution on the electrode is not influenced by adjacent electrodes because of the large distance between the electrodes. As a consequence, the charge distribution is circumferentially uniform, field enhancement effects occur near the ends of the electrodes, and the excess charge distribution near the ends of the electrodes causes the current to flow in a vertical direction rather than in a direction between the electrodes themselves. As a consequence, only the volume immediately surrounding the electrode is heated to a significant degree.
What is needed is an improved ohmic heating array design, wherein the flow of currents and related heating into formations of no interest is suppressed, and furthermore, a more even heating of the formation is achieved.